Display systems have become more complex in recent years. Television receivers, for example, have a greatly increased number of functions available to the user. These functions may include the routing and processing of stereo sound signals, the selection of various auxiliary video sources and the scheduling of future programs. Typically, in order to utilize these functions, a relatively large number of function selection switches are required. This makes the input device such as the hand-held remote control transmitter large, difficult and confusing to use and expensive to manufacture.
One way to minimize the number of switches on the remote control unit is to provide a display of a matrix of functions displayed on the screen of the display system in what is sometimes called a menu, and to provide input selection apparatus for selecting which ones of the displayed functions are to be exercised. Menu based systems for use in television receivers are disclosed in U.S. Pat. No. 4,641,205 (Beyers) issued 3 Feb. 1987 and U.S. Pat. No. 4,626,892 (Nortrup et al.) issued 2 Dec. 1986, both of which are assigned to the same assignee as the present application. The input device may be a so-called "mouse", a joy stick or a light pen, each of which provides X-axis and Y-axis position information to a display controller for determining the desired function. Typically, the user must use both hands or must have a flat surface on which to move the device. Most of these devices are wired to the controller and therefore do not lend themselves to remote control at distances usually associated with television viewing. It is desirable, however, that an input device make use of the human propensity to point at a desired item, thus providing ease of operation. Light pens and touchscreens provide such a pointing function but necessarily require the user to be within an arm's length of the display screen.
A wireless input device used for pointing to a particular item to be selected from a menu displayed on a display screen is disclosed in U.S. Pat. No. 4,565,999 (King et al.) issued on Jan. 21, 1986. This prior system determines the angular position of the remote transmitter unit by comparing the relative amplitudes of the "pointing" signals sequentially received by the display device.
In some applications, such as remotely controlling a television receiver, the user, and hence the remote control handunit, may be separated from the television receiver by as little as 3 feet and by as much as 30 feet (with a consequent 20 db signal amplitude variation). Thus, a system using amplitude modulated signals encounters difficulty in preserving the amplitude-proportionality of the samples, through the detection, preamplification, sampling and holding processes. Requiring the system to operate over the aforementioned distances, places severe linearity and stability constraints on the receiver circuitry. Any nonlinearity at the higher signal amplitudes (short distance) or instability at the lower signal amplitudes (long distance) will degrade the ability of the system to accurately convey user "pointing" information to its destination. Specifically, the AGC function of the signal amplifier circuits may be difficult to implement.